SAN FRANCISCO — Officials say a wildfire in a rural part of Northern California’s Napa County that has damaged two homes and forced the evacuation of 200 others grew overnight, as crews struggled to get a handle on the flames in the steep terrain.

The fire had burned more than 6 1/2 square miles as of Thursday morning and grown 500 acres from the previous day. At 4,300 acres, it remained 30 percent contained.

State fire spokeswoman Alicia Amaro said the fire made short runs uphill during the night.

The steep and rugged terrain has forced crews to build containment lines by hand without the help of bulldozers.

The fire has damaged two homes and seven outbuildings.

Besides the homes that have been evacuated, 180 more are threatened, but it was burning to the north of the county’s famous vineyards.

“It has not come anywhere close to what we consider Napa Valley wineries,” said Cate Conniff, a spokeswoman for the Napa Valley Vintners, a nonprofit trade association. “It is moving in the opposite direction, and it continues to move that way. We’re keeping an eye out on it.” Pope Valley is about 20 miles north of Napa Valley.

More than 1,000 firefighters were hoping to get a better handle on the blaze after they made progress overnight Tuesday, said Daniel Berlant, a spokesman for the California Department of Forestry and Fire Protection.

But fire activity increased about noon Wednesday as temperatures again soared into the 90s and the blaze continued burning northeast into neighboring Lake County.

Crews have requested more bulldozers from across the state to assist the dozen on site in strengthening containment lines, Berlant said.

No injuries have been reported, and the cause of the fire remains unknown.

The fast-moving blaze began Tuesday afternoon in Napa County. Within hours, it covered 600 acres and then spiked to 2,700 acres by late evening as it spread northeast. State firefighters and crews from Napa, Lake and Solano counties spent a second day working in 90-degree weather on Wednesday.

Similar high temperatures were expected again on Thursday. The fire exploded because of dry conditions across the state, Berlant said.

Gov. Jerry Brown said Wednesday that he has secured federal funds to help fire departments absorb some of the cost of fighting the fire.

Elsewhere in California, firefighters were surrounding a blaze in Shasta-Trinity National Forest that forced the evacuation of about 15 homes. Forest spokeswoman Andrea Capps said the fire has burned through 35 acres since it started Tuesday and was 70 percent contained.

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